It was years later when he was old had had more wrinkles than he cared to know about (and after he had gotten over the shock of living so long) that he began to wonder and think. Falling in love with Nami had been like getting shot. He had thought it might happen, braced himself in case it did, was shocked when it did happen, and then realized he had no idea what to do about it.
When had he fallen for her in the first place? There were so many little things that he loved about her, so many big things, so many things he couldn't stand, things that drove him crazy, but he fell completely and utterly, head over feet, spinning head, pounding heart absolutely in love with her.
But Zoro couldn't put his finger exactly on when it had happened.
Maybe it was when she put out the fuse with her bare hands. Surely it hadn't happened that fast. They had only just met then. He stopped Buggy's men from attacking her, but only because he had seen her brace for the strike and Luffy was in no position to help her.
Maybe it was when she threatened to drown their captain for leaving her "hard earned" money behind and got a laugh out of him.
Maybe it was when Luffy, for the first time ever, handed his hat over willingly to another person. After only a few days of knowing her, Luffy had trusted her enough to let her repair his most prized possession. She had known how much trust and faith had been put in that gesture, he knew she did. But she had just smiled and threaded her needle.
Maybe it was when she slipped the food she didn't want onto their plates so smoothly even Sanji didn't notice. They ate more than she did, so she kept asking for more and kept letting it vanish off her plate. It might have even been when she asked for rum to drink because she knew he liked it.
Maybe it was when she stole Merry. The anger he felt after that had been more than he'd felt in a long time.
Maybe it was when he realized she was one of Arlong's crew, but not by choice. Her voice was snide and her composure was calm, but her eyes were blank and pained and he knew there had to be more than what she was telling.
Maybe it was when she beat him in front of those freaky fish people, because that had actually hurt.
Maybe it was when he saw her pull a knife on the villagers to try and keep them from charging the park. The same knife she used to carve the ink from her arm. The same knife she would have used to kill herself if Luffy hadn't stopped her. He realized then just how strong Nami really was, how desperate and over her head she was, how much she needed their help.
Maybe it was when she looked out to sea as they sailed away and she smiled and said she was finally free.
Maybe it was when they were being turned into candles. Despite the fear in her eyes, she was bickering with him like always, like nothing was wrong. She played up her fear a little too much and he saw through it to the faith she had in the rest of the crew. She wasn't truly afraid because she knew if he couldn't get them out of this situation, the others could.
Maybe it was when she got sick and passed out on the ship. That had been terrifying. Enemies threatening Nami could be taken out with swords, but a virus needed a doctor, and his swords were useless.
Maybe it was in Alabasta when he saw her limping her way back to him, leaning on her staff as her foot dragged uselessly behind her, smearing blood in the dirt as she hobbled along. She broke into a smile when she saw him, trying to hide the wound by putting weight on it. He saw the way she flinched and carried her without too much of a protest even though they both knew his wounds were worse.
Maybe it was when, as they left a tearful Vivi on the shore, Nami turned and bared the symbol first to let the princess know that the pirates would always be her friends.
Maybe it was at Christmas that first year when they got caught under the mistletoe when no one was around. He told her she didn't have to, but she did anyways. That had been when he felt the first inkling of knowledge, when he first realized what could happen between them if he let it. But he knew he didn't deserve her, so he shoved it aside and lost himself in a sweaty afternoon of training.
Maybe it was at that bar. He forgot the name of it a long time ago, but he clearly remembered the pretty boy trying to grope Nami at every turn. Nami had eventually lost her patience and, after three warnings, smacked him silly. The dumbass thought he'd pull a gun on her, but Zoro had pulled his swords and made him rethink putting his hands on her ever again.
Maybe it was when he realized that he was always the first to her side at even a hint of danger.
It could have been when he decided he didn't have to outright torture himself and allowed himself to take her hand one night, making the excuse that she'd get lost without him. She had laughed at him and maneuvered her hand in his so that their fingers were interlocked. His heartbeat didn't slow back down until he went to sleep that night.
Maybe it was the day he woke up from a mid-afternoon nap to find Nami sprawled in the chair beside his, head slumped over just enough to rest on his shoulder as she dozed in the warm sun. Zoro indulged himself that day and took two naps just so he could fall asleep surrounded by her scent.
He knew he was in a lot of trouble when she threw her arms around his neck and hugged him for all she was worth despite all the dust and blood that coated him. He knew he was doomed when he realized he didn't even have it in him to not hug her back. He called himself weak but he still couldn't let go.
He knew it was all over when he realized she cared for him differently than the others. Naturally she hadn't said anything, which made him wonder how he knew, but he'd always known things about Nami, picked up on subtle hints given by her eyes or posture that the others didn't seem to notice.
He knew he was done for when he confronted her about everything and she said she knew how important his training was. She said she knew that nothing could get in the way of his dream, not even a relationship. She said she understood that and that they should go about their lives like nothing was different.
Maybe it was when he kissed her before she could make any more excuses.
Zoro watched his wife bustle about the house with excitement, cleaning everything so their grandchildren could destroy everything they touched. Her hair was mostly white with only streaks of red left. She had laugh lines around her mouth and wrinkles at the corners of her eyes. She had to wear her glasses all the time now, and it was getting harder and harder for her to hold a pen. She was still beautiful and still loved him, proving it by dropping a kiss on his head as she passed.
He didn't know exactly when he had fallen for Nami. He'd probably never know. But as his children and grandchildren came crashing through the house, somehow louder than Luffy had ever been, Zoro realized that it didn't matter because he'd fallen for her and, for some reason, she had fallen for him too and they'd had a wonderful, crazy, insane, perfect life because of it.